Some molecular studies we had been doing for the identification of rapidly growing mycobacteria led us to examine more carefully the parameters involved in the carbon source utilization test for the identification of these organisms. This phenotypic test has been considered useful for distinguishing among certain species of clinically significant rapid growers, especially in those institutions that do not have the capability for molecular analysis. However, our data suggest that carbon source utilization does not always allow definitive discrimination among certain species. This study is nearing completion, and a manuscript summarizing our findings will be prepared.